Pressure tanks serve two useful purposes. First, when pumps move water, they tend to create a pulsing effect on the flow. So when you go to turn on your faucet, the water can sort of jiggle back and forth, which can be annoying. A pressure tank eliminates the pulsing effect. The second reason is that the tank reduces the number of on/off cycles of the pump. With a properly sized pressure tank, the tank will be able to handle enough volume of water and pressure such that the pump won't have to turn on every time one turns the water on. For example, one could wash one's hands and the amount of water it would take would only reduce the pressure a slight amount and the pump wouldn't need to turn on. By reducing the amount of on/off cycles for the pump, the life of the pump is extended.